A pellicle is a thin film for protecting a mask, which is used to pattern a layer on a semiconductor substrate, from airborne contamination, such as dust, so that the airborne contamination does not cause the pattern to be misprinted on the semiconductor substrate. Since the pellicle undesirably collects dust, the pellicle needs to be far enough away (e.g., a couple of millimeters) from the reticle so that the dust and other particles on the pellicle do not print. To avoid affecting the photolithography process, the pellicle is a transparent film.
As light travels through the mask and then through the pellicle, light is reflected at the first interface between the environment (external reflection) and the pellicle, and some of it is reflected within the pellicle (internal reflection). The materials and thickness for the pellicle are chosen so that the internal and external reflections of light travelling normal to the pellicle cancel each other so that light transmission is optimized.
Prior art pellicles used in semiconductor manufacturing are thin (i.e., thickness less than 3 um) organic polymer films. The prior art also contains research into thick (i.e., thickness greater than 600 um) inorganic substrates which provide mask contamination control and function optically as removable photolithographic lens elements. These thick inorganic substrates have been named “hard-pellicles” and are made from materials such as modified fused silica. Prior art pellicles are only optimized for light entering the pellicle at angles normal to the pellicle, although non-normal angles have been considered when designing lenses in photolithographic systems and for thick inorganic hard-pellicles. In addition, prior art pellicles have not included anti-reflective coatings, although such coatings have been used on a semiconductor wafer to decrease reflection during photolithographic patterning, on optical lens elements and have been considered for thick inorganic hard-pellicles. Furthermore, it is known that quarter wave plates are effective at optimizing transmitted light intensity in optical systems.
As semiconductor device features (e.g., transistor length) decrease in size, the angle of the light travelling through the pellicle increases. Thus, as semiconductor device feature sizes decrease the pellicles used for large device features are no longer optimized for smaller dimensions. Therefore, a method is needed for determining how to make pellicles that can be used for the smaller device features.